But the unspoken secret in the world of religion research is that no one knows for certain what Americans believe – or even what they do on Sunday mornings – because no one has figured out how to measure something as intimate and intangible as religious faith.

When it comes to assessing the health and future prospects of American Christianity – or of any other faith, for that matter, in one of the most religious nations on Earth – there's a lot of opinion and argument. But no one really knows.

Consider:

Last year, nearly three-quarters of Americans told the Pew Research Center they're Christian. Forty percent described themselves as “born again” in a Gallup poll. The same number reported attending church the previous week.

That's 125 million Americans in church each Sunday, many of them born-again Christians. Pretty healthy, right?

Yes, but:

The number of Christians in the Pew poll actually represented a decline – down 5 percentage points from just five years earlier.

Those self-described “born-agains”? One-fifth never go to church, according to The Barna Group, a Christian research organization.

And there's no way 125 million Americans were listening to sermons last weekend, according to The American Church Research Project, which compiles national church attendance numbers.

Just 19 percent of Americans were in church on an average Sunday in 2007, according to a Research Project report published the following year. Cut that 125 million in half.

So, is Christianity healthy? Declining? Hanging in there?

‘Different measures'

“There are different narratives out there that are relying on different measures,” said Conrad Hackett, a Pew Research Center demographer. People “hear one number one day and another number the next week, and don't realize there's tension between those numbers.”

That might explain the loud debate about the health of one particularly influential and hard to measure branch of American religiosity: Evangelical Christianity.

Two much-discussed books published this year by prominent evangelical pastors – one of them a former investigative journalist – warn that evangelicalism is on the verge of steep decline, with members, money and cultural capital hemorrhaging.

Not so fast, say other evangelicals, including Orange County's own Rick Warren, pastor of Saddleback Church in Lake Forest. Evangelicalism, according to Warren and other defenders, is healthy. The doomsayers are misreading the polls.

Who's right? I'll attempt to answer that question next week.

But first, to understand how to answer the question, I need to take you on a side trip into the fascinating – and maddening – world of American religion research. You might wonder why such a basic question as the health of American Christianity is so difficult to answer. I can explain.

The answer begins exactly 56 years ago, in 1957. That's the year government officials floated the idea of asking Americans about their religious identity as part of the U.S. Census.

As you might imagine, there was a huge backlash, both from people alarmed at the prospect of bureaucrats poking into their religious beliefs, and from those committed to strict separation of church and state. After a trial run, census takers dropped the idea.

Ever since, knowledge about Americans' religious preferences has depended on an ad hoc group of university researchers, church organizations, private companies and polling foundations such as Pew Research.

Call it the privatization of religion research, with predictable results: A huge amount of information, none of it standardized, all of it susceptible to accidental or deliberate misinterpretation.

Take evangelicalism. In recent decades, evangelicals have been among the most prominent and influential religious groups in the nation. Yet, researchers trying to count them can't even agree on who is evangelical.

To Gallup, someone is evangelical if they self-identify as “born again” (41 percent of Americans in 2012).

But Pew uses a different measurement, taking into account respondents' church denomination. Because, Hackett said, it turns out a lot of people call themselves “born again” who don't really fit the evangelical mold.

In other words, people responding to surveys often don't know what “evangelical” means, either. (For the record, Pew counted 28 percent of Americans as evangelical last year.)

Which is why The Barna Group uses an even more restrictive definition, asking poll respondents a series of detailed questions about their beliefs and practices. By Barna's measure, fewer than 10 percent of Americans qualify as evangelical.

Is it any wonder people argue over the state of the American church?

Similar problems confront researchers of other faiths. The number of Muslims in the U.S. has been estimated at 1.3 million (American Religious Identification Survey, 2008); 2.6 million (Association of Religion Data Archives, 2010); and 7 million (Council on American-Islamic Relations).

As for how many people of faith live in a particular city or county – again, no one really knows, though several private organizations publish data based on various collection methods.

As a religion reporter, I've learned to be skeptical about every religion statistic I come across. And yet, such numbers are vital to my reporting and I use them all the time.

I'm especially skeptical of statistics produced by religious organizations themselves. Hackett pointed to an example from Brazil, where the Vatican recently estimated the number of Catholics at 163 million.

That's nearly 40 million more than the number counted by Brazil's census, which does ask about religion. Why the difference? Because the Vatican counts anyone baptized in a Catholic church as Catholic. But in Brazil, as in other developing nations, many baptized Catholics have been switching to Pentecostal and other Protestant churches.

Both figures are correct. Which one you use depends on the story you want to tell.

Don't give up on stats

Despite the contradictions, Hackett said people of faith – and anyone else interested in American religion – shouldn't give up on statistics.

“It would be a shame to take away that we can't believe anything,” he said. “There's good research going on and it can tell us something. We just have to keep in mind where the numbers come from.”

Among the worst offenders in this arena are people in my profession, journalism, who jump on isolated studies – such as a Pew report last year showing a rise in religiously unaffiliated Americans – to make broad, inaccurate generalizations.

Faith, like every other aspect of American life, is complex. People who want to know about it have to be patient to sort through conflicting information and make reasoned judgments.

That's what I'll try to do next week about the future of evangelical Christianity. The answer is out there, somewhere in the numbers. You just have to know how to find it.

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